GaN-Based Light-Emitting Diode With Three-Dimensional Silver Reflectors

We present a simple and robust method to fabricate three-dimensional Ag reflectors on GaN light-emitting diodes (LEDs) using SiO $_{2}$ nanospheres as the template. First, the hexagonal arrays of SiO$_{2}$ nanosphere monolayer were spun-cast on a benzocyclobutene (BCB) layer, which was prepared on a sapphire surface. Then, the bottom half of the SiO$_{2}$ nanospheres were embedded into the BCB layer after heating, resulting in arrays of “nano-lenses” that were in the shape of convex hemispheres. The concave-shaped hemisphere arrays were produced by etching the SiO$_{2}$ nanospheres with an HF solution. Ag was deposited onto both patterns, concave and convex hemispheres, resulting in the formation of three-dimensional Ag reflectors. From the electroluminescence measurements, these Ag reflectors, which contained either concave or convex hemisphere patterns, were found to enhance the light output of GaN LEDs by as much as 29%–33%.      

High-Efficiency GaN-Based Light-Emitting Diodes Fabricated With Metallic Hybrid Reflectors

We report on high-efficiency GaN-based light-emitting diodes (LEDs) fabricated with metallic hybrid reflectors (MHRs). It is shown that the MHRs consisting of an injection part (with low p-contact resistance and intermediate reflectance) and a spreading part (with high p-contact resistance and good reflectance) can enhance current injection and overall light reflectivity. Compared with reference LEDs with single reflectors, LEDs fabricated with Ag-based MHRs give higher output power by 8.9% and a reduction in forward voltage by 0.02 V (at 20 mA), resulting in the improvement of the power efficiency by 10%.     

Fabrication of Dicing-Free Vertical-Structured High-Power GaN-Based Light-Emitting Diodes With Selective Nickel Electroplating and Patterned Laser Liftoff Techniques

Through the use of selective nickel (Ni) electroplating, patterned laser liftoff technique, and surface roughing of the top n-GaN epilayer, a novel process for the fabrication of vertical-structured metal-substrate GaN-based light-emitting diodes (VM-LEDs) to avoid difficulties in Ni substrate dicing and improve device yield was proposed and demonstrated. In conjunction with a sidewall passivation with SiO₂ and keeping the size of epilayer smaller than that of Ni island, a considerable improvement in yield and device performance were shown. As compared to conventional lateral-structured GaN-based LEDs, VM-LEDs show an increase in light output power about 174% at 350 mA with a significant decrease in forward voltage from 3.5 to 3.17 V     

GaN-Based High-Q Vertical-Cavity Light-Emitting Diodes

We report a fabrication and demonstration of a GaN-based high-Q vertical-cavity light-emitting diode (VCLED). The GaN VCLED is composed of a 25-pair high-reflectivity (98%) GaN/AlN distributed Bragg reflector (DBR), an eight-pair SiO₂/Ta₂O₅ dielectric DBR (99%), and a three-lambda optical thickness InGaN/GaN active region. It shows a very narrow linewidth of 0.52 nm, corresponding to a cavity Q -value of 895 at a driving current of 10 mA and a dominant emission peak wavelength at 465.3 nm. In addition, this VCLED emission linewidth continues to decrease with an increasing injection current, suggesting a possible realization of GaN-based vertical-cavity surface emitting lasers.     

Phosphor-Free GaN-Based Transverse Junction White-Light Light-Emitting Diodes With Regrown n-Type Regions

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones.     

High Brightness GaN-Based Light-Emitting Diodes

This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces     

On a GaN-Based Light-Emitting Diode With a p-GaN/i-InGaN Superlattice Structure

An interesting GaN-based light-emitting diode (LED) with a ten-period i-InGaN/p-GaN (5-nm/5-nm) superlattice (SL) structure, inserted between a multiple-quantum-well structure and a p-GaN layer, is fabricated and studied. This inserted SL can be regarded as a confinement layer of holes to enhance the hole injection efficiency. As compared with a conventional LED device without the SL structure, the studied LED exhibits better current-spreading performance and an improved quality. The turn-on voltage, at 20 mA, is decreased from 3.32 to 3.14 V due to the reduced contact resistance as well as the more uniformity of carrier injection. A substantially reduced leakage current (10^-7-10^-9 A) and higher endurance of the reverse current pulse are found. As compared with the conventional LED without the SL structure, the significant enhancement of 25.4% in output power and the increment of 5% in external quantum efficiency are observed.     

High-Speed GaN-Based Green Light-Emitting Diodes With Partially n-Doped Active Layers and Current-Confined Apertures

We demonstrate a high-speed GaN-based green light-emitting diode for plastic optical fiber (POF) communication applications. By using a combination of n-type doping and undoped In_xGa_1-xN/GaN based multiple quantum wells (MQWs), and a 76-mum-diameter current-confined aperture structure, we can obtain an extremely high electrical-to-optical (E-O) 3 dB bandwidth (~330 MHz), which is limited by the spontaneous recombination lifetime of the MQWs. A reasonable coupled power (-264 muW) can be simultaneously achieved for a 2 mm in diameter POF with a 0.5 numerical aperture (NA).     

Improvement of Reliability of GaN-Based Light-Emitting Diodes by Selective Wet Etching With p-GaN

In an attempt to enhance the reliability of GaN-based light-emitting diodes (LEDs), the selective wet chemical etching of p-GaN surface in the GaN-based LEDs using KOH+NaOH in an ethylene glycol solution was investigated. The leakage currents of the etched LED under forward and reverse bias voltages were much lower, compared to those of a nonetched LED. The etched LED also showed improved light extraction efficiency and the degradation rate of light output power at a high injection current of 300 mA was slower than that for a nonetched LED. These results can be attributed to a decrease in the surface defects, an increase in hole concentration, and the increased surface roughness of the etched p-GaN     

Design Criteria for Near-Ultraviolet GaN-Based Light-Emitting Diodes

We discuss, through numerical device simulation, a number of possible design approaches intended for optimizing the internal quantum efficiency (IQE) of light-emitting diodes based on InGaN quantum wells (QWs) grown along the c-axis emitting in the near-ultraviolet region. We study the effects on IQE of thickness, doping, and alloy composition of the electron and hole blocking layers in order to maximize the confinement of both carrier species in the active region. We discuss the selection of the number of QWs to be employed in the active region and their optimum width, and we show the comparatively minor effects of the thickness of the barrier layers. We also compare different strategies for barrier doping, confirming that a p-type doping in all barriers helps to compensate the spontaneous and piezoelectric surface charges and to enhance hole transport. Finally, we evaluate the impact of Auger recombination on IQE and its role in the experimentally observed efficiency droop. Whenever possible, we suggest practical design criteria and provide technologically feasible sets of design parameters.     

Enhanced Light Extraction From Triangular GaN-Based Light-Emitting Diodes

This study investigated the characteristics of a triangular light-emitting diode (LED) and compared it to a standard quadrangular LED. The total radiant flux from the packaged triangular LED increased by 48% and 24% at input currents of 20 and 100 mA, respectively, compared to that of a quadrangular LED which was grown on patterned sapphire substrate. In light far-field beam distribution, the light extraction in the horizontal direction of the LED was much higher than that of the quadrangular LED due to the enhancement of light emission from the side walls of the triangular LED.     

大功率GaN基LED的提取效率

影响GaN基LED效率的主要因素是内量子效率和提取效率. 蓝光GaN基的LED内量子效率可达70%以上， 紫外GaN基LED可达80%，进一步改善的空间较小. 而传统大面积结构GaN基LED由于全反射和吸收等原因，外提取效率只有百分之几，提高空间很大. 本文从几何和物理光学角度分析了影响GaN基LED外提取效率的因素，针对全反射、吸收、横向光波导等问题总结了现有的各种提高GaN基LED提取效率的手段及其优缺点.     

大功率GaN基LED的提取效率

影响GaN基LED效率的主要因素是内量子效率和提取效率.蓝光GaN基的LED内量子效率可达70%以上,紫外GaN基LED可达80%,进一步改善的空间较小.而传统大面积结构GaN基LED由于全反射和吸收等原因,外提取效率只有百分之几,提高空间很大.本文从几何和物理光学角度分析了影响GaN基LED外提取效率的因素,针对全反射、吸收、横向光波导等问题总结了现有的各种提高GaN基LED提取效率的手段及其优缺点.     

Linear Cascade GaN-Based Green Light-Emitting Diodes With Invariant High-Speed/Power Performance Under High-Temperature Operation

We demonstrate the performance of linear cascade green light-emitting diode (LED) arrays suited for use in plastic optical fiber (POF) communications in automobiles or harsh environments. With this three-LED array, driven by the constant voltage bias of an in-car battery output (12 V) we obtain high-speed ($sim$100-Mb/s eye-opening), high-coupling power (0.9 mW), and a very small variation of coupled power versus temperature [$-$ 0.12%$^{circ}hbox{C}^{-1}$ at room temperature (RT)] for the whole measured temperature range (i.e., RT to 120 $^{circ}$ C). Even under high bias current (100 mA) operation, our device can sustain a clear 150-Mb/s eye-opening from RT to 120 $^{circ}$C. The static and dynamic measurement results indicate that the speed and power performance of this device are less sensitive to variations in ambient temperature than are those of the red resonant-cavity LEDs utilized for POF communication.      

垂直结构GaN基LEDs电流分布计算分析

电流分布是影响大功率LEDs器件性能的重要因素,与传统结构GaN基器件比较,垂直结构器件通过采用上下电极分布,明显改善了LEDs器件内部电流分布均匀性。通过理论分析与数值计算,建立起了垂直结构GaN基LEDs电流分布模型,研究了垂直结构GaN基LEDs电流分布及I-V特性。结果表明,与传统平面结构比较,垂直结构GaN基LEDs的电流分布均匀性得到了明显改善,同时正向电压降低约7%。最后,通过晶片键合与激光剥离技术,制备了垂直结构GaN基LEDs,测试结果表明,实验结果和理论计算值相吻合。该结果对GaN基LEDs器件的优化设计具有重要指导意义。     

Study of GaN-Based Light-Emitting Diodes Grown on Chemical Wet-Etching-Patterned Sapphire Substrate With V-Shaped Pits Roughening Surfaces

We investigate the mechanism responding for performance enhancement of gallium nitride (GaN)-based light-emitting diode (LED) grown on chemical wet-etching-patterned sapphire substrate (CWE-PSS) with V-Shaped pit features on the top surface. According to temperature-dependent photoluminescence (PL) measurement and the measured external quantum efficiency, the structure can simultaneously enhance both internal quantum efficiency and light extraction efficiency. Comparing to devices grown on planar sapphire substrate, the threading dislocation defects of LED grown on CWE-PSS are reduced from 1.28 times 10⁹/cm² to 3.62 times 10⁸/cm², leading to a 12.5% enhancement in internal quantum efficiency. In terms of the theoretical computing of radiation patterns, the V-Shaped pits roughening surface can be thought of as a strong diffuser with paraboloidal autocorrelation function, increasing the escape probability of trapped photons and achieving a 20% enhancement in light extraction efficiency. Moreover, according to the measurement of optical diffraction power, CWE-PSS demonstrated superior guided light extraction efficiency than that of planar sapphire substrate, thus an extra 7.8% enhancement in light extraction efficiency was obtained. Therefore, comparing to the conventional LED, an overall 45% enhancement in integrated output power was achieved.     

GaN-Based Light-Emitting Diode Prepared on Nano-Inverted Pyramid GaN Template

Using self-aligned $hbox{SiO}_{2}$ nano-spheres as an etching mask, the authors demonstrated the formation of a GaN-based nano-inverted pyramid (NIP) structure. It was found that crystal quality of the GaN epilayer prepared on an NIP/GaN template was significantly better than that prepared with conventional low-temperature GaN nucleation layer. With the NIP structure, it was found that 20-mA light-emitting-diode (LED) output power can be enhanced by 32%, as compared with the conventional LED.      

图形化蓝宝石衬底GaN基LED的研究进展

蓝宝石衬底作为发光二极管最常用的衬底,经过不断发展,在克服其与GaN间晶格失配和热膨胀失配问题上,研究人员不断提出解决方案。近期发展起来的图形化衬底技术,除了能减少生长在蓝宝石衬底上GaN之间的差排缺陷,提高磊晶质量以解决失配问题,更能提高LED的出光效率。从衬底图形的形状、尺寸、制备工艺出发,回顾了图形化蓝宝石衬底GaN基LED的研究进展,详细介绍了近年来关于图形化衬底技术与其他技术在提高LED性能方面的结合,总结了图形化蓝宝石衬底应用于大尺寸芯片的优势,并对其未来在大功率照明市场的应用进行了展望。     

Enhancement of Light Extraction Efficiency Using Lozenge-Shaped GaN-Based Light-Emitting Diodes

We have demonstrated both theoretically and experimentally that a lozenge-shaped light-emitting diode (LED) enhances light extraction efficiency compared with a conventional rectangular LED. The total light output power of the lozenge-shaped LED on a transmitter optical can (TO-can)-type package shows an increase of 12% at an injection current of 20 mA when compared with that of a rectangular LED. Moreover, the series resistance and the forward voltage of the lozenge-shaped LED slightly decrease compared with those of the rectangular LED. The far-field emission pattern shows that the light escaping from the lozenge-shaped LED along the horizontal direction is larger than that from the rectangular LED.      

Electrical Characteristics of GaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.